Co-pending application Ser. No. 07/476,050 relates to improved microcellular carbon fibers based on polyacrylonitrile (PAN) and structures formed of such mircocellular carbon fibers from PAN precursor fibers, these being used as a replacement for carbon fibers made from high purity viscose rayon and especially for use in the space industry.
However, carbon fibers are used in many environments in addition to the space industry as disclosed in parent application Ser. No. 07/476,050, and carbon fibers of this type are made from various precursor materials including pitch. In general, carbon fibers are used in what may be broken down into three general categories, namely in thermal insulators, in structural applications, and in miscellaneous environments.
Thus, conventional carbon fibers are used in thermal insulation environments to replace asbestos for many purposes, such as furnace insulation, brakes including aircraft, automotive, truck, and off-road vehicle brakes, passive fire protection, etc. In brakes, carbon fibers are used in a carbon matrix to provide a carbon-carbon structure.
Concerning carbon-carbon materials, the Kirk-Othmer Encyclopedia of Chemical Technology (3rd Ed. 1980), Vol. 12, page 463 states:
Carbon represents the ultimate high temperature end-member of polymer matrix materials. It has one of the highest temperature capabilities under non-oxidizing conditions among known materials (it melts or sublimes, depending on the pressure, at 3550.degree. C.). Additional considerations of chemical and thermal compatibility make it natural to use carbon and graphite fibers as the reinforcement material. The resultant carbon-carbon . . . composites . . . are especially desirable where extreme temperatures may be encountered, such as in rocket nozzle, ablative materials for re-entry vehicles and disk brakes for aircraft. Other uses include bearing materials . . . and hot-press die components. PA1 The decrease in specific gravity of Novoloid-based carbon as temperature exceeds 1000.degree. C. is remarkable and has not been adequately explained. It does not appear related to any observable development of microporosity or voids. PA1 Thermoplastic matrix materials are expected to assume a major role in fiber-reinforced plastics in the next few years. Thermoplastics have the potential for reduced fabrication costs, improved repairability, damage tolerance, and chemical resistance. However, development of an inexpensive thermoplastic that adheres well to carbon fibers and has satisfactory resistance to solvent has not yet to be achieved.
The specific gravity of carbon fibers depends on a number of factors including the nature of the precursor material and the degree of crystallinity (if any) in the resultant carbon fiber. Thus, well-ordered graphite molecular structure is dense. Novoloid precursor based carbon fibers are amorphous and have a relatively low specific gravity, whereas carbon fibers based on PAN are much denser having a normal specific gravity (g/cm.sup.3) of 1.8-2.0. The Kirk-Othmer Encyclopedia of Chemical Technology (3rd Ed. 1981), Vol. 16, page 135 contains a table (Table 3) showing typical properties of carbon fibers.
This table is reproduced below:
TABLE 3 __________________________________________________________________________ Typical Properties of Carbon Fibers Precursor Property Novoloid Pitch Polyacrylonitrile __________________________________________________________________________ type low modulus low modulus high modulus treatment temperature, .degree.C. 800 2000 1000 2000 1500 2000 specific gravity, g/cm.sup.3 1.55 1.37 1.63 1.55 1.8-1.9 1.9-2.0 carbon content, wt % 95 99.8+ 95 99.5+ 93 99.5+ x-ray diffraction profile, 002,20 23.0.sup.a 25.0.sup.a 24.0.sup.a 25.0.sup.b 26.1.sup.c degrees interlayer spacing, d.sub.002, pm 395 351 336 tensile strength, MPa.sup.d 500-700 400-600 500-1000 1500-3000 elongation, % 2.0-3.0 1.5-2.5 1.5-2.5 1.0-1.5 modulus, GPa.sup.e 20-30 15-20 30-50 150-300 heat resistance, .degree.C. tga 436 541 416 519 air 350 380 350 350 specific resistivity, m.OMEGA.-cm 10-30 5-10 10-30 1-10 afiinity with PTFE, CPE, epoxides.sup.f good fair poor __________________________________________________________________________ .sup.a Broad. .sup.b Medium. .sup.c Sharp. .sup.d To convert MPa to psi, multiply by 145. .sup.e To convert GPa to psi, multiply by 145,000. .sup.f PTFE = polytetrafluoroethylene; CPE = chlorinated polyethylene.
The text on page 135 states:
The most commonly used PAN based carbon filaments have a specific gravity of 1.75 (e.g. Hercules AS-4), whereas the most commonly used pitch based carbon filaments, mesophase pitch based, have a specific gravity of 1.85 to 2.10.
In structural applications, there is an important relationship between the weight of the fiber and its strength. Carbon fibers are often used in place of glass fibers as reinforcement in order to save weight, for example in aircraft and space structure where weight is critical. In aircraft, carbon fiber is used for reinforcement of secondary structures and interior parts such as flooring, luggage bins, ducting, etc. While conventional carbon fibers are very useful in the environments noted above and have an excellent strength to weight ratio, the need exists for fibrous reinforcing materials having an even better strength to weight ratio.
Carbon fibers are also used in a variety of miscellaneous environments such as for high temperature gaskets, seals, pump packing, medical implants, cement reinforcement, etc. Most fiber-reinforced plastics are laminated materials. The fibers in each layer are usually arranged in one of four configurations such as in the form of uni-directional tape, woven fabric, chopped and aligned fibers or randomly disposed fibers in the form of a mat or non-woven fabric.
As pointed out above, two important factors in any use are strength and weight. Another important property is the compatibility of the carbon fibers to the matrix material, e.g. how well the matrix material adheres to and holds the carbon fiber reinforcing material in place. Carbon fibers do not always adhere as well as desirable to the selected matrix material. Conventional carbon fibers especially do not adhere well to thermoplastic resins. Thus, the Kirk-Othmer Encyclopedia of Chemical Technology, (3rd Ed., 1984) Supplement Volume states:
Accordingly, improvements in these areas would be desirable.